Remove dependency on Flutter; upgrade dependencies

[michaelstepner]

In the jmap-dart-client library, Flutter is only used to execute tests. This PR removes the dependency on flutter and replaces testing using flutter test with dart test, using the dart test package: https://pub.dev/packages/test

The modified code files have also been linted by the default Dart linter, which has generated a large number of whitespace changes in the diff. If you'd like me to revert the whitespace changes and limit the diff to meaningful changes, just let me know. (Done!)

Motivation: I'm trying to use this library in a Dart codebase that does not use Flutter, and Flutter is a heavy dependency which was causing package versioning conflicts. Upon investigation, I realized the jmap-dart-client package isn't using any flutter-specific features, which makes sense, since it is a backend library and flutter is an SDK for frontend UI.

[hoangdat]

Hi @michaelstepner, thanks for your contribution.
Your change also includes a lot of format change (dart linter). Can we eliminate it?

Thanks and BRs

[michaelstepner]

@hoangdat Thanks for reviewing, I've eliminated the formatting changes in the latest commit to this PR.

[michaelstepner]

@dab246 the CI failures should be resolved in commit a0a16df

[dab246]

@dab246 the CI failures should be resolved in commit a0a16df

Still failed. Please check log

[michaelstepner]

@dab246 would you prefer to upgrade to v3 of the Dart SDK, or downgrade to an older version of the test package? I installed v2.19 in the ci.yml Github Action because the existing copy of pubspec.yaml requests an outdated version of the Dart SDK:

environment:
  sdk: ">=2.19.2 <3.0.0"

In a separate branch of my fork (update-dependency-versions), I upgraded the Dart SDK and all packages to their latest versions, and verified that the tests run successfully.

[michaelstepner]

would you prefer to upgrade to v3 of the Dart SDK, or downgrade to an older version of the test package?

Commit 2e0c3e8 upgrades the Dart SDK to v3 and upgrades the package dependencies to their latest versions. Commit ec04a1c does so without the linter changes. I've verified that all the tests pass locally:

$ dart test
Building package executable... (1.8s)
Built test:test.
00:02 +84: All tests passed!

@dab246 you can test again in the Github Actions CI system, or let me know if you'd prefer a different approach.